Method and apparatus for producing zinc-aluminum alloy-coated steel sheet with superior workability and corrosion resistance

ABSTRACT

Disclosed is a method for producing a zinc-aluminum-based alloy-coated steel sheet with superior workability and corrosion resistance by coating a base zinc-aluminum-based alloy-coated steel sheet in a coating bath comprising 35 to 55% by weight of zinc, 0.5 to 3% by weight of silicon, 0.005 to 1.0% by weight of chromium, 0.01 to 3.0% by weight of magnesium, 0.001 to 0.1% by weight of titanium, and the balance of aluminum and inevitable impurities.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and apparatus for producing a zinc-aluminum alloy-coated steel sheet with superior workability and corrosion resistance.

2. Description of the Related Art

A zinc-coated steel sheet is widely used based on economic efficiency and resource abundance in an attempt to secure corrosion resistance of a base steel sheet and is the most generally used coated-steel sheet. In addition, a great deal of research to improve the corrosion resistance of zinc-coated steel sheets has been made. In particular, an aluminum-coated steel sheet (so-called “Galvalume”) having an Al—Zn content of 55% was suggested in the late 1960s and exhibits superior corrosion resistance and a beautiful appearance at present.

Such an aluminum-coated steel sheet exhibits superior corrosion resistance and heat resistance, as compared to zinc-coated steel sheets and is thus widely applied to automobile mufflers, household appliances, heat-resistant materials and the like.

For example, Japanese Patent Publication No. 57-47861 discloses an aluminum steel sheet containing Ti in iron, Japanese Patent Publication No. 63-184043 discloses an aluminum-coated steel sheet containing C, Si, Cu, Ni and a small amount of Cr in iron, and Japanese Patent Publication No. 60-243258 discloses an aluminum-coated steel sheet containing 0.01 to 4.0% of manganese, 0.001 to 1.5% of titanium and 3.0 to 15.0% of silicon.

In addition, in order to inhibit growth of a Fe—Al alloy layer or rapid diffusion of aluminum metal into iron by reaction of aluminum with iron, 10% or less of Si is added to an aluminum coating bath. A coated steel sheet produced by this method exhibits relatively superior workability and heat resistance and is widely used for heat-resistant elements such as automobile mufflers, hot water suppliers, heaters, and electric rice cooker inner skins.

However, silicon added to inhibit formation of alloy layers may often cause damage to surface appearance of coated steel sheets and disadvantageously make the surface appearance unclear. In this regard, damage to surface appearance caused by silicon addition is known to be solvable to some extent through addition of a small amount of magnesium (U.S. Pat. No. 3,055,771 to Sprowl).

In addition, in recent years, extended lifespan of components used for automobile exhaust gas systems has brought about development of steel sheets obtained by introducing Cr to an aluminum-coated steel sheet. For example, Japanese Patent Publication No. 63-18043 discloses a coated steel sheet containing 1.8 to 3.0% of chromium and Japanese Patent Publication No. 63-47456 discloses a steel sheet containing 2 to 3% of chromium.

Meanwhile, a Zn—Al alloy-coated steel sheet has a disadvantage in that a processed shear portion does not exert sufficient corrosion resistance. This phenomenon is caused by deterioration in corrosion resistance of a surface exposed to the shear portion which results from a decrease in sacrificial corrosion-resistant zinc preventing corrosion of iron through the zinc-aluminum alloy layer. In addition, a Zn—Al alloy-coated steel sheet has a disadvantage of deterioration in corrosion resistance after processing since a coating layer having no heterogeneous alloy phase is formed and an interface surface is vulnerable upon use after a bending or drawing processing and corrosion resistance is thus deteriorated after the processing.

In order to solve these phenomena, Korean Patent No. 0586437 discloses coating a Zn—Al—Mg—Si alloy-coated steel sheet material with superior corrosion resistance in a coating bath containing 45 to 70% by weight of Al, 3 to 10% by weight of Mg, 3 to 10% by weight of Si, and the balance of Zn and inevitable impurities, and Korean Patent No. 0928804 discloses a Zn—Al—Mg alloy-coated steel sheet with superior corrosion resistance and workability.

SUMMARY OF THE INVENTION

Therefore, the present invention has been made in view of the above problems, and it is one object of the present invention to provide a method for coating a zinc-aluminum-based alloy-coated steel sheet which further improves workability and corrosion resistance by simultaneously adding suitable contents of chromium, magnesium and titanium to a conventional zinc-aluminum-silicon coating bath.

It is another object of the present invention to provide a method for coating a zinc-aluminum-based alloy-coated steel sheet which inhibits formation of a MgO oxide film on the surface of the coated steel sheet by adding a predetermined content of calcium to the coating bath.

It is yet another object of the present invention to provide a device for forming a nitrogen cloud (tent) which further inhibits formation of an oxide film on the surface of a coated steel sheet by surrounding the circumference of the coated steel sheet ascending from the surface of a coating bath with nitrogen gas.

In accordance with the present invention, the above and other objects can be accomplished by the provision of a method for producing a zinc-aluminum-based alloy-coated steel sheet with superior workability and corrosion resistance, the method comprising coating a base zinc-aluminum-based alloy-coated steel sheet in a coating bath containing 35 to 55% by weight of zinc, 0.5 to 3% by weight of silicon, 0.005 to 1.0% by weight of chromium, 0.01 to 3.0% by weight of magnesium, 0.001 to 0.1% by weight of titanium, and the balance of aluminum and inevitable impurities.

In accordance with another aspect of the present invention, provided is a method for producing a zinc-aluminum-based alloy-coated steel sheet which inhibits formation of a MgO oxide film on the surface of the coated steel sheet by further adding 1 to 10% by weight of calcium, based on the total weight of magnesium, to the coating bath.

The coating layer may form a combination of a Mg₂Si phase, an Al phase, a Zn phase, a MgZn₂ phase, an Al—Zn—Mg—Ca mixed phase, an Al—Zn—Si—Cr—Ca mixed phase and an AlCr₂ phase on a typical surface structure of a general galvalume coated steel sheet.

Meanwhile, an amount of adhered coating is preferably set to 20 to 100 g/m² (on a basis of one surface), while taking into consideration deterioration in corrosion resistance caused by small amount of adhered coating and thus unstable development of grains, and low economic efficiency caused by large amount of adhered coating.

In addition, a temperature of the coating bath is 550 to 650° C. and a cooling speed after coating is controlled to 15 to 30° C./sec.

In accordance with another aspect of the present invention, provided is a device for forming a nitrogen cloud which further inhibits an oxide film on the surface of the coated steel sheet by surrounding the circumference of the coated steel sheet which ascends from the surface of the coating bath with nitrogen gas.

The device of the present invention is disposed between a surface of a coating bath to produce a zinc-aluminum-based alloy-coated steel sheet and an air knife, and forms a nitrogen cloud in a circumference of the coated steel sheet ascending from the coating bath.

The device may include lower nitrogen supply bars and spaced from the surface of the coating bath by a predetermined distance, the lower nitrogen supply bars discharging nitrogen gas in a direction of the surface of the coating bath along the circumference of the coated steel sheet, a side cover extends upward at an angle from a side of the lower nitrogen supply bars in a direction of the coated steel sheet, and upper nitrogen supply bars formed on the upper surface of the side cover and discharging the nitrogen gas downward.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is images (magnification of 1,000×) showing surface of Galvalume-based Mg and Mg—Cr—Ca coated steel sheets;

FIG. 2 is images (magnification of 2,000×) showing the cross-section of a Galvalume-based Mg and Mg—Cr—Ca coated steel sheets;

FIG. 3 is a schematic view illustrating inhibition of formation of MgO oxide film caused by addition of Galvalume-based Mg and Mg—Cr—Ca;

FIG. 4 is a plan schematic view illustrating a device for forming a nitrogen dam (device for forming a nitrogen cloud);

FIG. 5 is a sectional view taken along A-A′ section of FIG. 4; and

FIG. 6 is a schematic view of the device shown in FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the present invention will be described in more detail.

In the method of the present invention, a coating bath contains 35 to 55% by weight of zinc. Zinc has superior sacrificial corrosion resistance to iron and thus functions to inhibit corrosion. It is necessary to secure 35% by weight or more of zinc. This is because when the zinc content is lower than this range, a temperature of the coating bath increases, top dross thus increases and workability is lowered which has negative effects on operation. In addition, when the zinc content is 55% by weight or more, production costs increase and economic efficiency is deteriorated due to increase in specific gravity of the coated steel sheet.

The coating bath of the present invention contains 0.5 to 3.0% by weight of silicon. Silicon functions to inhibit growth of an alloy layer, is effective in improving flowability of the coating bath and imparting gloss thereto and should be added in an amount of 0.5% by weight or more. A key role of silicon in the coating layer is to control formation of an alloy layer of a base steel sheet with aluminum. When an amount of added silicone is 0.5% by weight or less, the function of silicon is limited and workability is considerably deteriorated. On the other hand, when silicone is added in an amount exceeding 3% by weight, Mg₂Si serving as a factor which contributes to improvement of corrosion resistance of the coating layer is excessively produced and grown on the surface of the coating layer, the surface thereof is rough, thus causing surface discoloration at an early stage and deteriorating post-treatment coating properties. Accordingly, the amount of added silicon is preferably 0.5 to 3% by weight.

Chromium added to the coating bath functions to form a dense and passive oxide film on the surface of the coating layer, improve corrosion resistance of the aluminum-coated steel sheet and make grains of the coating layer fine since chromium is uniformly distributed in the coating bath.

Chromium functions to form a predetermined shape of an Al—Zn—Si—Cr mixed phase band integrated in the coating layer (FIG. 2). Chromium present in the coating layer reacts with aluminum to form an AlCr₂ phase and functions to improve workability and corrosion resistance at a fracture plane after processing. The chromium enables the content of silicon to be controlled to 3% by weight or less and prevents thus excessive precipitation of silicon in the form of a needle in the coating layer.

The content of chromium providing these effects is known to be 0.1 or more (U.S. Pat. No. 3,055,771 to Sprowl). However, in the method of the present invention, the content of chromium is 0.005 to 1.0% by weight. When the content of chromium is 0.005% by weight or less, chromium is not readily homogeneously distributed in the coating bath, and when the content thereof is 1.0% by weight or more, an increase in temperature of the coating bath is required due to increase in chromium content, dross increases and appearance is disadvantageously damaged due to the dross attached to the surface of the coated steel sheet.

Accordingly, the content of chromium is preferably 0.005 to 1.0% by weight.

The coating bath of the present invention also contains 0.01 to 3.0% by weight of magnesium.

Magnesium added together with chromium is bonded to oxygen present in the air contacting the coating layer to form a passive film and thus prevents oxygen from diffusing into the alloy layer, and prevents additional corrosion and thus improves corrosion resistance. Presence of a Mg₂Si phase (see FIGS. 1 and 2) formed through reaction between magnesium and silicon, and a MgZn₂ phase produced through reaction between magnesium and zinc in the coating layer functions to reduce corrosion speed through sacrificial corrosion resistance of zinc during corrosion and formation of a local battery. In addition, magnesium reacts with aluminum, blocks permeation of oxygen and thus considerably improves corrosion resistance of a shear surface.

When an amount of added magnesium is 0.01% by weight or less, dispersibility, and improvement effect of corrosion resistance associated with oxidation properties are low, and when the amount exceeds 3.0% by weight, the coating bath is saturated, a melting point increases, workability is deteriorated, surface qualities are deteriorated due to continuous generation of dross on the surface, production costs increase and problems associated with production processes become serious.

The amount of added magnesium is preferably 0.01 to 3.0% by weight.

The coating bath of the present invention also contains calcium in an amount of 1 to 10% by weight with respect to the weight of magnesium. Calcium added together with magnesium and chromium inhibits formation of magnesium oxide on the interface of a coated melt metal and thus prevents deterioration in appearance qualities by a fine magnesium oxide film adhered to the surface of the coated steel sheet.

Addition of Ca, Be, Al or the like to a molten Mg bath is known to considerably inhibit oxidation and combustion of the molten Mg bath even at a high temperature. In accordance with a mechanism which inhibits oxidation of the molten Mg bath through addition of calcium, a combustion temperature of the molten Mg bath increases to 200° C. or higher due to addition of calcium. This increase in combustion temperature of a Mg alloy generally causes an oxide layer generally formed on the surface to be changed from a porous oxide layer to a dense oxide layer and efficiently blocks permeation of oxygen.

When the content of calcium is 1% by weight or less with respect to the weight of magnesium, dispersibility is deteriorated and inhibition effect of the MgO oxide film are low, and when the content thereof exceeds 10% by weight with respect to the weight of magnesium, deterioration in coating layer workability caused by formation of a metallic compound of aluminum and calcium may occur. Accordingly, the amount of added calcium is preferably 1 to 10% by weight with respect to the weight of magnesium.

The present invention provides application of a nitrogen spray nozzle-attached dam which enables nitrogen purging and prevents an oxide film from being adhered to strips, to a lower surface of an air knife of the coating bath. Formation of an oxide film is inhibited by purging the lower surface of the air knife which ascends to the interface of the coating bath after a strip is dipped in the coating bath, with a nitrogen atmosphere and nitrogen wiping is performed on the lower surface of the nitrogen dam through a nitrogen curtain nozzle in order to prevent introduction of a fine oxide film formed after contacting the air in an outside of the surface of the molten coating bath into the dam and adhesion thereof to the strip.

Furthermore, the coating bath of the present invention also contains 0.001 to 0.1% by weight of titanium in order to reduce size of spangles which constitute appearance of the coating layer and form a flower shape of the coating layer. When an amount of added titanium is 0.001% by weight or less, dispersibility on the steel sheet is deteriorated, and when the amount thereof is 0.1% by weight or more, dissolution in the coating bath is not easy and titanium does not affect improvement in effects.

The present invention is based on size reduction of spangles which realizes by increasing likelihood of nucleation on a conventional Galvalume coated steel sheet through addition of suitable amounts of chromium, magnesium, calcium and titanium to a coating bath containing aluminum, zinc and silicon.

That is, added components are dispersed in the coating layer to form various nuclei such as Mg₂Si, MgZn₂ and AlCr₂ phases after the steel sheet is coated, and mutual interference between grain boundaries controls growth of the grains.

Accordingly, beautiful surface appearance is secured, corrosion between the grain boundaries is inhibited and corrosion resistance is enhanced. In addition, growth of an alloy layer of aluminum and iron is inhibited and a coating film with superior workability is thus formed.

Meanwhile, it is preferable to set a temperature of the base steel sheet bathing in the molten coating bath to 570 to 650° C. and a temperature of the molten coating bath to 550 to 650° C.

When the bathing temperature of the base steel sheet is lower than 550° C., flowability of the coating bath is deteriorated, appearance of coating is bad and coating adhesiveness is deteriorated. When the bathing temperature thereof is 650° C. or higher, rapid thermal diffusion of the base steel sheet causes abnormal growth of the alloy layer and deterioration in workability and formation of excessive Fe oxide layer in the molten coating bath.

A coating amount is preferably 20 to 100 g/m², on a basis of one side. When the coating amount is 20 g/m² or less, air pressure of the air knife equipment controlling the coating amount excessively increases, variation in coating amount occurs, and damage to appearance of the film and adhesion of oxide dross thereto occur due to rapid increase in surface oxide in the molten coating bath.

In addition, when the coating amount is 100 g/m² or more, the alloy layer is excessive formed and workability is considerably deteriorated.

Meanwhile, the present invention provides a device for forming a nitrogen film which further inhibits an oxide film on the surface of the coated steel sheet by surrounding the circumference of the coated steel sheet which ascends from the surface of the coating bath with nitrogen gas.

FIGS. 4 to 6 are schematic views illustrating a device according to the present invention.

The device of the present invention is spaced from the surface of a coating bath 3 by a predetermined distance and moves upward and downward between the surface of the coating bath 3 and an air knife 2 by a lifting means 5.

The device of the present invention includes lower nitrogen supply bars 41 and 42 formed in the form of a rectangle along the circumference of the coated steel sheet 1 ascending from the surface of the coating bath 3. The lower nitrogen supply bars 41 and 42 receive nitrogen from the nitrogen supply pipe 46 disposed at a side thereof and discharge nitrogen gas toward the surface of the coating bath 3. Although not shown, the lower nitrogen supply bars 41 and 42 are provided at a lower surface thereof with a plurality of holes (nozzles) disposed by a predetermined distance, to enable discharge of nitrogen gas.

The lower nitrogen supply bars 41 and 42 are rectangular pipes which may be integrally formed. As shown in FIG. 4, the first bar 41 and the second bar 42 are branched and separated from each other and are thus spaced from each other in a width direction (in a longitudinal direction in the drawing).

In addition, the device of the present invention includes a side cover 43 which extends upward at an angle from the side of the lower nitrogen supply bars 41 and 42 toward the coated steel sheet 1 and upper nitrogen supply bars 44 and 45 which are formed on the upper surface of the side cover 43 and discharge nitrogen gas 10 downward.

The upper nitrogen supply bars 44 and 45 are pipes which are provided with nitrogen discharge holes (not shown) formed toward the surface of the coating bath and face each other on the upper surface of the side cover 43 and discharge nitrogen gas inwardly. The upper nitrogen supply bars 44 and 45 receive nitrogen from the nitrogen supply pipe 46.

Meanwhile, the side cover 43 extends upward at an angle from the lower nitrogen supply bars 41 and 42 to the upper nitrogen supply bars 44 and 45 toward the coated steel sheet 1. As a result, the discharged nitrogen gas 10 is not diffused and is captured around the coated steel sheet 1.

By forming a nitrogen cloud 47 around the relatively hot coated steel sheet 1 ascending from the surface of the coating bath 3 using the device for forming the nitrogen cloud according to the present invention, formation of oxide film on the surface of the coating bath 3 can be inhibited.

Hereinafter, the present invention will be described in more detail through comparison between Examples and comparative Examples. These examples are provided only to illustrate the present invention in more detail and should not be construed as limiting the scope and spirit of the present invention.

A cold-rolled steel sheet with a thickness of 0.8 mm, a width of 120 mm and a length of 250 mm was coated using a melt-coating simulator. As shown in Table 1, a zinc-aluminum-based alloy-coated steel sheet was produced by changing a composition of the coating bath. In addition, a nitrogen cloud was formed using the nitrogen cloud formation device shown in FIGS. 4 to 6.

The amount of adhered coating was controlled using an air knife and the amount of coating of the produced zinc-aluminum-based alloy-coated steel sheet evaluated based on one side is shown in Table 1.

Evaluation items were corrosion resistance and workability. Corrosion resistance was compared with an initial rust generation time (5%) under a 35° C. NaCl salt spray test atmosphere in accordance with KSD 9504 and evaluated. Workability was compared and evaluated by observing a width (fracture width) of cracks generated after 180° OT bending test in accordance with a KSD 0006 test method using a 30 to 50× stereomicroscope and measuring the width of the fracture surface. Observation of alloy phase was carried out using an X-ray diffraction.

Detailed test results obtained by the test method are given below.

1. Workability: observed according to crack width level.

⊚: fracture width of 10 to 20 μm

Δ: fracture width of 20 to 30 μm

×: fracture width of 40 to 50 μm

2. Dross level: an amount of dross generated in an upper part of coating bath after molten coating specimens according to coating composition.

⊚: generation of 5% or less of dross with respect to coating bath

Δ: generation of 10 to 20% less of dross with respect to coating bath

×: generation of 20% or more of dross with respect to coating bath

3. Surface appearance: visibility (clearance) and formation level of spangles of surface appearance of coating layer observed by the naked eye

⊚: Clear formation of spangles with high gloss

Δ: Non-clear formation of spangles

×: Little formation of spangles with bad appearance

4. Corrosion resistance of shear surface: ratio of rust generated after salt spray test for 1,000 hours

⊚: rust ratio of 5% or less

Δ: rust ratio of 10 to 20%

×: rust ratio of 30% or more

5. Corrosion resistance of flat portion: a ratio of rust generated after salt spray test for 2,500 hours.

⊚: rust ratio of 5% or less

Δ: rust ratio of 20 to 30%

×: rust ratio of 30% or more

TABLE 1 Amount Coating Appln. Formation of bath Cooling of of alloy Evaluation of Coating bath composition (% by weight) coating Temp. speed nitrogen phase physical properties* Items Zn Si Cr Mg Ca Ti Balance (g/m²) (° C.) (° C./sec) dam Mg₂Si AlCr₂ A B C D E Exams. of 1 43.4 0.5 0.05 0.01 0.001 0.001 Al and 75 600 25 Applied X X Δ ⊚ Δ X Δ present 2 42.0 1.6 0.1 1.0 0.15 0.01 impurity 60 590 20 Applied 0 0 Δ ⊚ ⊚ ⊚ ⊚ invention 3 35.0 1.6 0.3 1.5 0.05 0.1 50 600 30 Applied 0 0 ⊚ ⊚ ⊚ ⊚ ⊚ 4 42.5 1.0 0.5 3.0 0.01 — 20 610 15 Applied 0 0 ⊚ Δ ⊚ ⊚ X 5 40.5 1.6 0.5 1.5 0.1 0.001 40 600 30 Applied 0 0 ⊚ ⊚ ⊚ ⊚ Δ 6 40.4 1.6 0.3 2.7 0.02 — 60 630 20 Applied 0 0 ⊚ Δ ⊚ ⊚ ⊚ 7 55.0 1.6 0.3 1.5 0.1 0.001 35 600 15 Applied 0 0 ⊚ ⊚ ⊚ ⊚ X 8 43.4 1.6 0.3 1.5 0.1 0.05 60 600 20 Applied 0 0 ⊚ ⊚ ⊚ ⊚ ⊚ 9 40.7 1.6 — 2.7 0.3 — 50 590 30 Applied 0 0 Δ ⊚ ⊚ ⊚ ⊚ 10 45.0 3.0 0.3 1.5 0.05 0.001 100 600 25 Applied 0 0 ⊚ ⊚ ⊚ ⊚ ⊚ 11 434 1.6 0.3 1.5 0.1 0.05 60 600 20 Not 0 0 ⊚ ⊚ Δ ⊚ ⊚ applied Comp. 1 43.4 1.6 — — — — 75 600 25 X X X ⊚ ⊚ X Δ Exams. 2 42.0 1.6 — 4.0 — — 60 600 25 0 X X X ⊚ ⊚ ⊚ 3 40.0 1.6 — 5.0 — — 50 590 20 0 X X X ⊚ ⊚ ⊚ *A = Workability (fracture width), B = Generation of dross, C = Surface appearance, D = Corrosion resistance of shear portion, E = Corrosion resistance of flat portion

As can be seen from Table 1, the examples of the present invention exhibited superior workability and corrosion resistance. That is, Examples of the present invention exhibited a crack (fracture surface) generated after 180° OT bending, of about 10 to about 20 μm and thus superior corrosion resistance, as compared to comparative Examples. Examples of the present invention exhibited generation of a total cross-section rust after 3,000 hours or longer in an amount of adhered coating of 50 g/m2 on a basis of one side and generation of rust on the cross-section after 1,000 hours or longer. These results demonstrate that Examples of the present invention exhibit superior corrosion resistance, as compared to conventional compositions.

As a result of observation by the naked eye, Examples exhibited superior surface appearance, as compared to conventional Examples. This is caused by reduction of spangle size.

According to the present invention, a zinc-aluminum alloy-coated steel sheet with superior workability and corrosion resistance can be produced by controlling the size of an intermetallic compound layer and grains through suitable control of coating bath components.

In addition, by adding a suitable content of Ca to a coating bath and surrounding a circumference of a coated steel sheet ascending from the coating bath with a nitrogen cloud, problems including formation of an oxide film made of MgO on the surface of the coated steel sheet is inhibited and deterioration in surface qualities of the coating layer are solved.

Accordingly, the present invention is expected to be widely used in fields requiring corrosion resistance such as internal and external structural materials, household components and heat-resistant materials.

Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. 

What is claimed is:
 1. A device for forming a nitrogen cloud in a circumference of a coated steel sheet ascending from a coating bath, the device being disposed between the coating bath surface to produce a zinc-aluminum-based alloy-coated steel sheet and an air knife, the device comprising: lower nitrogen supply bars spaced from the surface of the coating bath by a predetermined distance, the lower nitrogen supply bars discharging nitrogen gas in a direction of the surface of the coating bath along the circumference of the coated steel sheet; a side cover extending upward at an angle from a side of the lower nitrogen supply bars in a direction of the coated steel sheet; and upper nitrogen supply bars formed on the upper surface of the side cover and discharging the nitrogen gas downward. 